In some types of memory technologies, data is stored by setting the state of a resistive element that may have varying resistance depending on the state. Examples of such memory technologies include magneto-resistive random access memory (MRAM) and resistive random access memory (RRAM or alternately called ReRAM). MRAM is a non-volatile random access memory technology that uses magnetic storage elements to store data. Spin-transfer torque MRAM stores data at memory cells having two superimposed layers of magnetic material separated by a thin insulating film, defining a magnetic tunnel junction (“MTJ” or “MTJ element”) of an MRAM cell. The two layers include a magnetic layer that is permanently magnetized in a fixed magnetic field alignment direction (this layer is referred to as a pinned layer), and a changeably-magnetized magnetic layer (this layer is referred to as a free layer). The changeably-magnetized magnetic layer can be magnetized in one of two orientations relative to the permanently magnetized layer. The two orientations are characterized by distinctly different serial resistances through the superimposed layers of the MTJ. The magnetic field orientation of the changeable layer can be aligned the same as that of the permanent magnet layer (parallel), or the magnetic field of the changeable layer can be aligned directly opposite to that of the permanent magnet layer (anti-parallel). The parallel alignment state has a relatively lower resistance and the anti-parallel alignment state has a higher resistance. These two states as sensed from their relatively higher or lower resistances (RH and RL) represent different binary logic values of bits in the memory.
For an MRAM write operation, a voltage VMTJ is applied, with one polarity or the reversed polarity, to the MTJ of an MRAM cell. Write lines referred to as a “bit line” and a “source line” may be used for providing VMTJ with either polarity. The bit line is coupled to the MTJ of each memory cell, and the source line is selectively coupled to each MTJ by a corresponding access transistor which may be turned on by a corresponding word line. If the MTJ of an MRAM cell is initially in the high resistance state, the low resistance state can be written by setting the bit line to a logical high voltage, setting the source line to a logical low voltage, and asserting a word line associated with the MRAM cell. Asserting the word line generates a bias voltage of a first polarity across the MTJ, and the MTJ changes over time to the parallel alignment (low resistance state). To write the low resistance state, the opposite voltage polarity is applied to the MTJ (e.g., the bit line is set low, and the source line is set high).
RRAM is another non-volatile memory type that uses a dielectric, which is normally insulating, and which can be made to conduct through a filament or conduction path formed after application of a suitable voltage. Once the conduction path is formed, it may be broken (resulting in a relatively high resistance) or re-formed (resulting in a relatively low resistance) by an appropriate voltage. In RRAM, the high or low resistance state may be determined by the direction of current flow.